1. Field of the Invention
This invention relates to olefin polymerization catalysts and methods and, more particularly, the invention relates to supported olefin polymerization catalysts having multiple catalytic sites that are capable of producing product polymer having broad or multimodal molecular weight distributions in a single reactor, and processes for making such polymers.
2. Description of Related Technology
Polyolefins, such as high density polyethylene (HDPE), having broad, and preferably multimodal (e.g. bimodal) molecular weight distributions ("MWDs") are useful for high molecular weight film and blow molding applications. Prior systems for the preparation of such polymers have utilized multiple reactors or catalysts which comprise mixtures of individual catalysts intended to produce polymers of differing molecular weights.
In multi-reactor systems, the olefin or olefins are sequentially polymerized in reactors containing catalysts known to produce polymers having relatively low and relatively high molecular weights, in an attempt to produce a product polymer mix having a broad or multimodal molecular weight distribution.
In some cases, attempts have been made to produce a broad or multimodal MWD product polymer in a single reactor using a mixture of two (or more) supported catalysts, each of which is known to produce a polymer having a target average molecular weight which differs significantly from that produced by the other of the catalysts. These attempts have met with only limited success for a variety of reasons.
One characteristic of prior mixed catalyst systems is that polymers of different molecular weights are formed by separate catalyst particles, which may limit mixing of polymers formed by such systems.